1. Field of the Invention
The present invention relates generally to a device which provides controllable levels of complex impedance and more particularly to a layered material with complex impedance properties for controlled transmission, reflection and/or absorption of selective frequencies of electromagnetic radiation.
2. Description of the Prior Art
Conventional structures utilized in the control of transmission, reflection and absorption of electromagnetic radiation typically incorporate one or more impedance devices to obtain the desired electromagnetic wave manipulation characteristics. The required impedance properties are generally characterized in terms of their resistive, inductive and capacitive components. These prior art impedance devices are typically fabricated in a sheet form and may comprise a layer of conductive or resistive material to obtain the desired impedance properties. One approach to achieve the desired level of impedance comprises the use of arrays or patterns of impedance elements which are formed from conductive or resistive materials on one surface of a layer of electrically insulating supporting material. The elements are typically formed by deposition of or by removing geometric sections from an electrically conductive or resistive film applied to the supporting material layer. The required capacitance of the impedance sheet may be controlled by varying the size and shape of the impedance elements, and by varying the spacing between the impedance elements in the array.
However, for applications requiring relatively higher capacitance values, typically sheet capacitance values on the order of 100 pico-Farads per square, this approach necessitates impedance elements of substantial proportions, leading to undesirable performance characteristics, including unacceptable levels of electromagnetic wave reflection. Also, when a composite structure is constructed from a plurality of impedance sheets incorporating such larger impedance elements which are spaced and separated by layers of dielectric material, the secondary capacitive effect created by the interaction between successive layers of impedance sheets in relative proximity can induce unforeseen and undesirable results in the electromagnetic wave manipulation properties of the structure.
One method which attempts to achieve greater capacitance while minimizing the secondary capacitive effect is the use of successive layers of conventional impedance sheets to form a thin composite device with multiple layers of impedance elements in relative proximity. This configuration induces a capacitive effect between proximate impedance elements in adjacent layers. By selecting appropriate sizes and shapes of the impedance elements, desired capacitance values can be achieved.
However, this approach also has serious limitations due to the inherent difficulty in maintaining the alignment between the elements in the successive layers. Minor misalignment of the impedance elements in successive layers can produce substantial localized variations in the capacitance properties of the sheet. Additionally, the cumulative effect of manufacturing tolerances over extended segments of the combined layers exacerbates the extent of impedance element misalignment and the resulting fluctuation of capacitance values. Further, large capacitance values can only be achieved by using layers of three or more impedance sheets, thereby increasing the thickness, weight and cost of the material, as well as increasing the potential for misalignment between successive layers within the combination configuration.
The general use of such geometrically shaped elements comprising an impedance layer of an electromagnetic radiation attenuation structure in the prior art is disclosed, by way of example, in U.S. Pat. Nos. 5,627,541; 5,576,710; 5,325,094; 5,214,432; 3,887,920; and 3,152,328. U.S. Pat. No. 5,627,541 uses an array of elongated, narrow conductive areas arranged in uniformly spaced columns and rows. U.S. Pat. No. 5,576,710 uses a series of conductive dipoles arranged in a semi-random or comparable pattern, preferably a series of square patches of particular dimensions separated by gaps of particular widths. U.S. Pat. No. 5,325,094 uses a resistive sheet formed into a broken pattern that may comprise a series of geometric shapes spaced apart from one other. U.S. Pat. No. 5,214,432 uses resistively loaded impedance elements that are disposed in a random and preferably a periodic pattern. U.S. Pat. No. 3,887,920 uses uniform geometric figures, including a thin film array of closely-packed aluminum squares. U.S. Pat. No. 3,152,328 uses layers of concentric printed disks of electrical energy absorbing material with incrementally reduced diameters, arranged in superposition to form a plurality of cone-shaped absorbing bodies.
The present invention eliminates or substantially reduces the disadvantages of the prior art through the use of conductive, resistive and/or inductive impedance elements on both sides of the dielectric sheet, rather than limiting their use to only one side. This configuration enables substantially larger values of capacitance to be attained in a single layer device, because relatively thin dielectric sheets can be used. Also capacitance can be precisely controlled. By varying the size, shape and composition of the impedance elements and by combining with resistive sheets, the desired combination of resistance, capacitance and inductive properties can be achieved to produce a tailored frequency-dependent electromagnetic wave reflection response.